Silanol-functional, branched organopolysiloxanes have numerous uses, for example as release coatings, and as curable and/or crosslinkable components in silicone compositions, such as organopolysiloxane coatings and elastomers.
Silanol-functional, branched organopolysiloxanes, which are predominately liquids, should not be confused with highly crosslinked organopolysiloxane resins, such as the well known MQ resins, which are generally of relatively high molecular weight, highly three-dimensionally and randomly cross-linked systems. Rather, silanol-functional, branched organopolysiloxanes are substantially star-like or comb-like polymers having, in general, rather lengthy silanol-functional and substantially linear organopolysiloxane chains emanating from a limited number of branching points.
The synthesis of such star- or comb-like polymers has been fraught with difficulty. For example, as recently as 1993, in U.S. Pat. No. 5,256,741, it is reported that there had been no reports of any star-type organopolysiloxanes having an SiO.sub.4/2 group as the branch center, and bonded at this branch center to one end of diorganopolysiloxane components. In the '741 patent, branched, but highly crosslinked organopolysiloxanes were prepared by treating organopolysiloxane resins containing chlorosilane moieties with n-butyl lithium, followed by reaction with a cyclic diorganopolysiloxane in ring-opening polymerization.
However, both the starting materials and end products of the '741 patent are poorly defined. The products are generally highly three dimensionally crosslinked resinous solids with a limited number of relatively long diorganopolysiloxane chains bonded to this highly crosslinked structure. Ratios of long chain diorganopolysiloxane moieties to SiO.sub.4/2 crosslinking/branching sites is limited to approximately 1:1; for example (SiO.sub.4/2).sub.22 (Me.sub.2 ASiO.sub.1/2).sub.20, where A represents a linear, n-butyl, methyl, vinyl siloxy-terminated polydimethylsiloxane containing 180 repeating dimethylsiloxy units. Such products have limited usefulness. Their method of preparation is quite expensive, and employs spontaneously flammable reactants such as n-butyl lithium. Silanol-functional products are said to be obtainable only in the presence of basic trapping agents for the HCl generated in the process.
In U.S. Pat. No. 5,254,658, silanol-functional MQ resins are dissolved in solvent and converted to silanolates with n-butyl lithium. These compounds are then reacted with cyclic organopolysiloxanes in ring-opening polymerization to form linear organopolysiloxane chains at the silanolate sites, the reaction stopped by adding end-blocking triorganohalosilanes such as dimethylvinylchlorosilane. Stopping the reaction with a protic acid results in silanol-functional chains. As with the '741 patent, the products are best described as MQ resin "nuclei" or "clusters" functionalized with long organopolysiloxane chains rather than true "star" or "comb" polymers. The products, being solids, have only limited usefulness. When used in release coatings, for example, the products cannot be used neat, but must be dissolved in solvent or other liquid components.
It would be desirable to prepare silanol-functional star- and comb-type polyorganosiloxanes with well defined structures which may, if desired, be substantially free of three-dimensionally cross-linked "nuclei" or "clusters"; which may be prepared as liquids, allowing use neat; and which may be prepared simply and without resort to dangerous reagents such as alkali metal alkyls.